Symptoms

• Is your system experiencing intermittent problems or acting "flaky"?
• Is your system hanging or locking-up after installing SCSI devices?
• Is there inconsistency to the number of SCSI devices that show up each time system is booted?
• Is your system experiencing read or write errors?

Most problems that occur during installation result from errors in preparing and connecting devices on the SCSI chain.

SCSI Errors can be intermittent and hard to chase down. A system with an improperly terminated SCSI bus may boot up fine and work. However, you may experience errors, hang-ups and data loss.

All AMS products are fully tested and inspected for proper SCSI configuration and device functionality. Before trying to change any of the settings inside your AMS product please review the following information on SCSI troubleshooting.

SCSI Basics

The following is a basic overview of SCSI technology.

SCSI
Small Computer Systems Interface. SCSI is a computer bus interface standard that allows a variety of computer peripheral devices such as disk drives, CD-ROM drives and tape drives to interface to a host computer.

SCSI ID
A SCSI ID is a unique address number ranging from 0 to 7 (0 to 15 for Wide SCSI). The SCSI ID identifies the SCSI device on the bus and sets priority. The higher the SCSI ID # the higher the priority when negotiating for the SCSI bus. Each device on a SCSI bus must have a unique SCSI ID.

Logical Unit Number (LUN)
Logical Unit Numbers are software references (Sub IDs) that allow 7 SCSI devices to share a single SCSI ID number. LUNs have always been a part of the ANSI SCSI specification and are supported by most SCSI host adapters.

SCSI Bus
The SCSI bus is the wiring used to connect SCSI devices to a host computer. This wiring includes the external cables, connectors and internal ribbon cables.

• A Narrow SCSI Bus is an 8 bit bus and can have up to 8 SCSI devices connected with the host adapter counting as one.
• A Wide SCSI Bus is a 16 bit bus and can have up to 16 SCSI devices connected with the host adapter counting as one.

SCSI Chain
A SCSI chain is one or more SCSI devices connected to each other with a cable and connector. A SCSI device can be outside (external) or inside (internal) your host computer. The SCSI host adapter is a SCSI device.

Termination
Termination closes the electrical circuit at each end of the SCSI chain and maintains signal integrity along the SCSI bus. Termination clears the noise out of the SCSI bus so that the SCSI bus can send and receive data more reliably. The basic rule of termination is that the first and last devices on a SCSI chain must be terminated. The SCSI devices in the middle must have termination disabled.

Termination Power
Termination Power is what drives termination in a chain of SCSI devices. The device supplying termination power can be anywhere on the bus.

SCSI Performance and Cable Length Specification

SCSI Troubleshooting Tips

Take a logical approach to the situation.
Don't start pulling things apart or making any changes before you have developed a plan of attack. SCSI is a very logical technology and a firm understanding is necessary in order to properly troubleshoot a problem situation.  Please review the SCSI Basics before you begin.

Make only one change at a time.
The only way to properly troubleshoot a situation is to change one thing at a time to see if that cures the problem.  If it does not cure the problem you should reverse the change before making another change.

Document each step.
Document each step you take in troubleshooting a SCSI problem. It will be worth it to find a clean sheet of paper and write down every step you take and what the results are. It helps to know what didn't work when determining your next step.

SCSI Troubleshooting Steps

The following is a list of things to check to ensure a proper SCSI configuration:

Are the SCSI cables properly and securely connected?
A loose cable connection can cause intermittent problems. External SCSI cables need to be securely fastened with the clips, thumbscrews or locking clamps built into the cables or connectors.

You may also want to check to see if any of the cables have bent pins. If you find a bent pin you have probably found your problem.

Are you using the cables supplied by AMS?
There are SCSI cables on the market that don't meet SCSI specifications. These can cause all sorts of SCSI problems.

Does each SCSI device on each SCSI chain have a unique SCSI ID?
Each device on the SCSI chain must have a unique SCSI ID. Make sure you know the SCSI ID of your SCSI host adapter. Most SCSI host adapters are factory default at SCSI ID 7.

If you are missing a SCSI ID during boot-up, you may have a SCSI ID conflict.

Is the active terminator placed on the last device in the SCSI chain?
The first and last device on a SCSI chain must be terminated. All devices in the middle must have termination disabled. An incorrectly terminated SCSI configuration can cause your system to hang.

Are you attaching an external Narrow SCSI (50 pin) device to a Wide SCSI (68 pin) bus populated with Wide SCSI devices?
If this is the case you need to terminate the unused lines or the "high nine" (or "high byte"). To do this you need a 68 to 50 pin external SCSI cable with built-in termination for the unused lines. Or you can use a 68 to 50 pin adapter with built-in termination for the unused lines and use a standard 50 to 50 pin external cable. Then you can terminate the external device with a 50 pin active terminator.

Are you attaching an internal Narrow SCSI (50 pin) device to a Wide SCSI (68 pin) bus populated with Wide SCSI devices?
If this is the case you need to use a 68 pin to 50 pin converter that blocks the High Byte (or high nine), but does not terminate it, because one of the rules of SCSI states that termination may only occur at the ends of the bus. The High Byte is blocked and only the Low Byte and Control signals are fed through the converter. The last device on this chain must be a wide device that is terminated or you must be using a 68 pin ribbon cable with an active terminator built in to or attached to the end.

Is your total SCSI cable length within the SCSI specification?
If your total cable length is too long you may not see all the devices on the SCSI chain or you may encounter SCSI errors. Add up the lengths of each external cable as well as the lengths of the cabling inside each SCSI device to see if you are within cable length specification. Refer to the SCSI specification chart for more detailed information.

Do you have many external daisy chain connections?
When multiple external SCSI devices are daisy-chained externally you may run into impedance problems.

Does your external device have a built-in termination switch?
If your external device has a termination switch built into the enclosure you may be double terminating which will definitely cause problems. If this device is the last device on the SCSI chain you can set the built-in terminator to "on" and you do not need an external terminator (if the built-in terminator is active). Or you can set the built-in terminator to "off" and use an external active terminator. Never set the built-in terminator to "on" and use an external terminator. Make sure the devices in the middle of the chain are not terminated.

Are you powering on the external SCSI devices prior to booting your system?
If you power on your system first and then power on the external SCSI devices you may not see all the devices.